Self-calibrating carbon dioxide analyzer

ABSTRACT

A carbon dioxide analyzer for medical purposes is rendered self-calibrating by continuously measuring pairs of two or three each of several components as follows: 
     1st pair: vacuum cell, open hole and sample cell reference cell, open hole and sample cell 
     2nd pair: vacuum cell, standard cell reference cell, standard cell 
     3rd pair: vacuum cell, open hole reference cell, open hole 
     4th pair: vacuum cell, standard cell, sample cell reference cell, standard cell, sample cell 
     The ratios of these measurement pairs are treated mathematically in a computer or microprocessor to obtain a reading for CO 2  and to correct other readings and to monitor the integrity of the standard cell. A novel two-wheel chopper and mirror arrangement facilitates the measurements.

TECHNICAL FIELD

This invention relates to the quantitative measurement of theconcentration of a particular gas in a gas mixture using the principleof non-linear absorption, by the gas to be measured, of particularwavelengths in the electromagnetic spectrum; for example, in the regionof infrared. The invention provides apparatus and method for suchmeasurements wherein the apparatus is continuously self-calibratingregardless of various temporal or environmental changes that can occurin the apparatus during normal operation and from time to time.

BACKGROUND OF THE PRIOR ART

This invention will be described as applied to the measurement thecarbon dioxide (CO₂). Such measurement apparatus is generally referredto as a gas analyzer and will be described as a carbon dioxide analyzer.The non-dispersive infrared technique utilizing the 4.26 micronsabsorption band of CO₂ has been widely used in the gas analyzer industryfor the detection of this gas. The term "non-dispersive" refers to theuse of spectral band-pass filtering. This technique offers a number ofadvantages including speed of response and greater sensitivity overolder methods that used the principle of heat transfer based uponradiation absorption by CO₂. However, this non-dispersive infraredtechnique is not immune to instrument zero and span drifts due tochanges incomponent characteristics caused by either aging or externallyinduced stimuli. Furthermore, this method does not automatically exploitthe other implicit advantages of this technique in achieving instrumentstability, interference rejection by other absorbing gases, and ease ofrecalibration.

NEGATIVE FILTERING

These inherent advantages of the non-dispersive infrared method can berealized by a technique known in the industry as "Negative Filtering"which combines CO₂ prefiltering or preabsorption and two-beam ratioing.In this technique two measuring conditions are set up. In the firstmeasuring condition, shown in FIG. A, also known as the "sample" beam,radiation from a blackbody source S is made to traverse a transparent"vacuum" cell V (or a cell that does not contain CO₂ gas) before passingthrough the sample chamber SC. The radiation then traverses theinterference band-pass filter F which spectrally limits the radiationbefore reaching the detector D, preferably a photodetector such as aPbSe photoconductor. The voltage transduced by the detector in thisconfiguration is usually referred to as V_(S), meaning the voltageresulting from the beam traversing a vacuum cell and the sample chamber.

In the second measuring condition shown in FIG. B, also known as the"reference" beam, radiation or light traverses a reference cell Rcontaining a known concentrated amount of CO₂ before passing through thesample chamber. The light then traverses the interference band-passfilter before reaching the PbSe detector. The transduced voltage by thedetector in this configuration is V_(R), meaning the voltage due toinfrared radiation passing through the reference cell and the samplechamber.

In the third measuring condition shown in FIG. C radiation traverses ahole before passing through the sample chamber. The light then traversesthe interference band-pass filter before reaching the PbSe detector. Thetransduced voltage by the detector in this configuration is V_(H),meaning the voltage due to the infrared radiation passing through thehole and the sample chamber.

THE RATIO V_(R) /V_(S) IS DESIGNATED AS "Q"

The voltage ratio V_(R) /V_(S) is used to relate the CO₂ gas present inthe sample chamber and is usually designated as Q. Because of thenon-linearity of CO₂ gas absorption at 4.26 microns, V_(S) decreasesmuch faster than V_(R) as a function of CO₂ gas present in the samplechamber. This relationship is shown in FIG. D. This is because in the"reference" beam the reference cell takes out most of the energycontained in the interference filter pass-band prior to its reaching thesample chamber. Thus, any additional CO₂ present in the sample chamberdoes not significantly alter this beam intensity. On the other hand, inthe "sample" beam the transduced voltage V_(S) varies more strongly withthe CO₂ gas present in the sample chamber (compared to V_(R)). This isbecause no absorption takes place in the vacuum cell, which does notcontain CO₂ gas. The ratio V_(R) /V_(S) therefore varies as a functionof CO₂ level in the sample chamber.

The ratio V_(R) /V_(S) or Q is used to correlate the measured CO₂ gasbecause of the fact that any nonspectral variation that is present inthe beam would appear to the detector to be the same for both beamsleading to substantial cancellation of the variation as seen by thedetector. These variations could be due to changes in any of the opticalcomponents of the system. For spectrally-related variations such asabsorption by interfering gases or change in the spectralcharacteristics for the source, filter, detector, etc., the cancellationis only operative to first order. Second or higher order corrections arenot operative even if the ratio V_(R) /V_(S) is used to relate the CO₂concentration in the sample chamber with the calibration curve. Thus, byusing a "Negative Filtering" technique, most first order effects causedby changes in the optical components such as aging or by externalstimuli are cancelled leading to a stable and accurate CO₂ measurementmethod.

THE RATIO V_(R) /V_(H) IS DESIGNATED AS "U"

Referring to FIGS. B and C, the voltage ratio V_(R) /V_(H) is used tomonitor the CO₂ gas level inside the sensor head or in the spacesbetween the various optical components from S to D. In order for thevoltage ratio Q=V_(R) /V_(S) to correctly relate the CO₂ gas present inthe sample chamber, as discussed previously, this CO₂ level inside thesensor head must in practice be very low (2 mmHg or less). When this CO₂level is very nearly zero the V_(H) curve (see FIG. D) is almostidentical (except for certain aperture effects due to differentgeometrical sizes which might be present) to the V_(S) curve and theratio U-curve follows closely to the ratio Q-curve as depicted in FIG.D. However, when the CO₂ level inside the sensor head starts to build upin quantity the output V_(H') (corresponding to the measurementarrangement shown in FIG. C) deviates substantially from V_(S) and theratio U'-curve (U'=V_(R) /V_(H')) is no longer the same as the originalQ-curve (see FIG. D). Significant error will result in the measurementof CO₂ gas present in the sample chamber if the user is not warned ofsuch a circumstance. The U-curve is used to provide such a warningsignal to the user should its difference from the original Q-curveexceeds a certain predetermined value.

DOUBLE-Q NEGATIVE FILTERING METHOD

Instead of using the voltage ratio V_(R) /V_(S) or Q to relate the CO₂gas present in the sample chamber a superior method commonly known tothe gas analyzer industry as the Double-Q Negative Filtering techniqueis preferred. With the selection of proper design values for the opticalcomponents used in the instrument such as in the case of theHewlett-Packard Capnometer this technique requires for recalibration thematching of only two points (commonly taken to be at 0 mmHg and 55 mmHg)on the calibration curve for adequately restoring the originalperformance accuracy for the instrument. In this technique two initialcalibration constants Q₀ and Q₁ are specially created for subsequentsignal processing use. Q₀ is defined as the value of Q=V_(R) /V_(S) whenthere is zero CO₂ present in the sample chamber. Q₁ is defined as thevalue of Q when there is 55 mmHg of CO₂ present in the sample chamber.

Both Q₀ and Q₁ are determined experimentally in the beginning and storedaway in non-volatile EAROM(Electrically Alterable Read Only Memory) inthe computer section of the instrument. They are so stored in order thattheir values can be quickly retrieved by the microprocessor which ispart of the instrument for computing the CO₂ value in the sample chamberbased upon the measured value of Q. Furthermore, both Q₀ and Q₁ valuescan be periodically checked for changes with the use of a zero CO₂standard (for Q₀) and a 55 mmHg CO₂ standard (for Q₁) in place of thesample chamber as practiced presently in the Hewlett-Packard Capnometer.The changes in Q₀ and Q₁ values, if any, reflect the component changesinside the instrument caused by either aging or external stimuli. Ifthese changes exceed a certain predetermined limit they can be appliedto the original values for Q₀ and Q₁ thereby generating new Q₀ and Q₁values. The fact that the original Q₀ and Q₁ values are stored in EAROMand not in the permanent memory (Read Only Memory) of the microprocessorenables the user to instruct the instrument to store these new values inplace of the old ones. The new or updated Q₀ and Q₁ values continue tobe stored in EAROM for further updating if needed in the future. Thesenew values of Q₀ and Q₁ adequately restore the measurement accuracy forthe instrument as so adeptly demonstrated in the current Hewlett-PackardCapnometer.

HOW THE DOUBLE-Q METHOD WORKS

The two-point recalibration routine described in the above section andafforded by the Double-Q technique is the direct result of using a newparameter called S (to be defined later) instead of Q in the calibrationcurve of the instrument. The former is called a calibration S-curve andthe latter a calibration Q-curve.

The pCO₂ in the sample chamber is represented by the calibration Q-curvein the form:

    pCO.sub.2 (mmHg)=B.sub.0 +B.sub.1 Q+B.sub.2 Q.sup.2

where

    Q=V.sub.R /V.sub.S

and B₀, B₁ and B₂ are coefficient constants empirically determined byfitting a large number of experimental data points linking pCO₂ levelsin the sample chamber to the measured Q values.

The pCO₂ in the sample chamber is represented by the calibration S-curvein the form:

    pCO.sub.2 (mmHg)=A.sub.0 +A.sub.1 S+A.sub.2 S.sup.2

where S is defined as

    S=(Q-Q.sub.0)/(Q.sub.1 -Q.sub.0)

where Q₀ and Q₁ are given by ##EQU1## as discussed previously. Thequantities A₀, A₁ and A₂ are coefficient constants empiricallydetermined based upon the experimental calibration curve of theinstrument, namely, pCO₂ in the sample chamber versus the measuredvalues of Q with the use of the calibration constants Q₀ and Q₁ asdefined above. The quantities A₀, A₁ and A₂ are stored in the permanentmemory (ROM) section of the microprocessor and their values therefore donot change with time. The quantities Q₀ and Q₁ are stored on the otherhand in EAROM and can be updated in time to reflect any changes in theoperating characteristics of the instrument. Since the instrument isbasically set up to measure Q (see FIGS. A and B) any changes occurringinside the instrument due to component aging or external stimuli will bereflected in the changes in measured Q values. If the pCO₂ in the samplechamber is represented by a calibration Q-curve, the pCO₂ value willchange leading to a measurement error. This is equivalent of saying thatthe original calibration curve has shifted. There is no simple way torestore the original calibration Q-curve except to recalibrate theinstrument afresh by taking many data points relating the pCO₂ values inthe sample chamber to Q values and redetermine a new set of coefficintconstants B₀ , B₁ and B₂.

However, if the pCO₂ in the sample chamber is represented by acalibration S-curve the same internal instrument changes which cause Qto change will also change the values of Q₀ and Q₁. It had beendemonstrated (for example in the Hewlett-Packard Capnometer) that inorder to adequately restore the original calibration S-curve in thiscase only the values of Q₀ and Q₁ need be updated without having toredetermine a new set of A₀, A₁ and A₂ values. This is apparent from thedefinition of S. The change in Q due to instrument aging or externalstimuli for the same pCO₂ value in the sample chamber is counteracted bysimilar changes in Q₀ and Q₁ so as to render the S value substantiallyunchanged. Since the set of A₀, A₁ and A₂ values remains unchanged(stored in ROM) the end result is that there will be negligible error inpCO₂ measurement despite the change in Q as long as the simultaneouschanges in Q₀ and Q₁ are noted and corrected.

BRIEF DESCRIPTION OF THE INVENTION

The principal drawback of these prior art gas analyzers using negativefiltering has been the operation to calibrate the instrument to give aneffective zero reading when no CO₂ is present in the sample chamber andto give an effective reading when a certain concentration is present.This has been done manually by substituting a vacuum cell for the samplechamber for an effective zero reading, and substituting a sealed vial ofknown gas concentration for the sample chamber to get calibration forthat standard concentration of gas. The present invention avoids thisnecessity for manual calibration, and provides a gas analyzer that iscontinuously self-calibrating during normal operation.

Apparatus embodying the invention eliminates the need to periodicallycheck the performance accuracy of the instrument and, in the event thatthe instrument is found to be operating out of performancespecifications, eliminates the need to manually perform therecalibration routine in order to restore its performance accuracy. Thecurrent invention renders the carbon dioxide analyzer continuouslyself-calibrating under normal operating conditions.

Another major improvement afforded by the present invention is theelimination of a relatively long warm-up time (typically 2-5 minutes)after initial instrument turn-on or during manual recalibration prior tothe instrument's accuracy being assured. Because of the continuouslyself-calibrating feature of the instrument provided by the presentinvention the warm-up time is significantly reduced to less than 30seconds.

A further advantage of the current invention is the elimination of theneed for a "zero" gas standard (commonly taken to be a sapphire blank)during the course of recalibration, thus reducing the cost of theinstrument.

It is a further object of the current invention to provide means forcontinuously checking the integrity of the calibration standard, whichtakes the form of a hermetically sealed transparent cell containing apredetermined amount of CO₂ gas such as 55 mmHg of CO₂. Such a means iscurrently not available in prior art. This is important in view of thefact that the absolute accuracy of the instrument depends upon the gasstandard being leak-tight. A continuous means for checking this standardcell for leakage guarantees recalibration accuracy.

It is a further object of the current invention to eliminate the need tostandardize the value of pCO₂ used in the "span" standard cell whereinpCO₂ denotes the partial pressure of CO₂ concentration. When thestandard cell is used externally for recalibration purposes such as inthe case of present day commercial CO₂ analyzers this standard celltends to be intermixed with other sensor heads and instruments. Normallythere is one sensor head associated with each carbon dioxide analyzer orinstrument. Each carbon dioxide commercial analyzer in turn stores onestandard cell for recalibration use. However, situation may arise suchthat different sensor heads are used with different instruments, andvice versa. Since the same zero and standard values of CO₂ have to beused in all present calibration analyzers the result is that each andevery standard cell stored in different instruments must have the samepCO₂ value. The values for these standard cells may of course differslightly but the difference directly translates into a recalibrationinaccuracy for all instruments. The current invention eliminates thisneed of exact standardizing of all the standard cells; i.e., they neednot have the same pCO₂ value. This greatly reduces the effective costfor these standard cells as a much larger percent of the manufacturedstandard cells can be used due to the tolerance relaxation in pCO₂value.

BRIEF DESCRIPTION OF THE INVENTION APPARATUS

The current invention comprises a combination of a unique dualwheeloptical system design and a special calibration methodology.

The special calibration methodology provides a mathematical formalismfor effectively restoring the original calibration curve of theinstruments by linking the outputs of two internal standards to systemchanges detected. One of the two internal standards is the normalambient environment inside the instrument. Since the ambient aircontains only a very small amount of CO₂ gas (typically 200 ppm) theambient environment inside the instrument is treated in a de factomanner as a zero CO₂ standard. Care must of course be taken not togenerate any CO₂ gas inside the instrument during the initialcalibration of the instrument. The other internal standard takes theform of a hermetically sealed standard cell with transparent windowscontaining an appropriate amount of CO₂ gas; for example, 55 mmHg, tosimulate an equivalence of 55 mmHg of pCO₂ flowing through the samplechamber of the instrument.

The unique optical system design affords the creation of eight specificmeasurements resulting in four ratios, sequentially in time andcontinuously. One of these measurement ratios is used to make the basiccarbon dioxide measurement using the double-Q negative filteringtechnique as currently employed, for example, in commercially availableCO₂ analyzers. The other three measurement ratios are employed to trackand detect environmental and system component changes including thebuild-up of carbon dioxide gas inside the instrument and the possibleleak of gas from the internal gas standard (55 mmHg). Environmental andsystem component changes detected by these three measurement conditions,or ratios, are used by the special calibration formalism or methodologymentioned to generate corrections in real time and applied to the carbondioxide measurement via the use of microcomputer and associatedelectronic circuitry. It is in this manner than the continuouslyself-calibrating feature of the present invention is achieved. At thesame time the need for a separate zero gas standard as used in presentcommercial instruments is eliminated. Furthermore, the need tostandardize the exact pCO₂ value for the standard cells for differentinstruments is rendered unnecessary. The current invention furtherprovides an automatic check for gas leaks in the standard cell notpreviously available in prior arts such as the Hewlett-Packard CO₂analyzer.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of apparatus embodying the invention.

FIG. 2 is a schematic diagram of the optical-mechanical apparatusembodying the invention and in which FIG. 2A is an enlargement of box 2of FIG. 1; FIG. 2B is a plan view of the left-hand chopper wheel of FIG.2A; and FIG. 2C is a plan view of the right-hand chopper wheel of FIG.2A;

FIG. 3 is a table showing the correlation between the quadrants of thetwo wheels of FIGS. 2B and 2C and the sample chamber, and naming thevoltage response of each combination;

FIG. 4A is a diagram of the wheels when they are aligned to pass radiantenergy through the vacuum cell, open hole, and sample cell.

FIG. 4B is a diagram of the wheels when they are aligned to pass radiantenergy through the reference cell, the open hole, and sample cell.

FIG. 5A is a diagram of the wheels when they are aligned to pass radiantenergy through the vacuum cell and standard cell.

FIG. 5B is a diagram of the wheels when they are aligned to pass radiantenergy through the reference cell and standard cell.

FIG. 6A is a diagram of the wheels when they are aligned to pass radiantenergy through the vacuum cell and open hole.

FIG. 6B is a diagram of the wheels when they are aligned to pass radiantenergy through the reference cell and open hole.

FIG. 7A is a diagram of the wheels when they are aligned to pass radiantenergy through the vacuum cell, standard cell, and sample cell.

FIG. 7B is a diagram of the wheels when they are aligned to pass radiantenergy through the reference cell, standard cell, and sample cell.

FIG. 8A is a diagram of the detector output voltage due to radiantenergy passing through the various combinations of vacuum cell,reference cell, open hole, and standard cell of FIGS. 4A, 4B, 5A, 5B,6A, 6B, 7A, and 7B, respectively.

FIG. 8B is a diagram of the sync detector output, with the double pulseindicating the beginning of the sequence of the various combinations, asshown in FIG. 3.

FIG. 8C is a diagram of the markings on the periphery of either one ofthe two wheels for the sync detector to generate the outputs shown inFIG. 8A.

FIG. 8D is a diagram of the signals for indicating the time to start thesampling of the detector outputs.

FIG. 8E is a diagram wherein the cross-hatched columns represent thetime during which the detector outputs are sampled.

FIG. 8F is a diagram of the voltage levels of the detector outputs.

FIG. 8G is a diagram showing the correlation of the markings on theperiphery of the wheel (FIG. 8C) with the various measurement numbers inthe lefthand column of FIG. 3.

FIGS. 9A through 9K are schematics and a chart showing prior art: FIG.9A showing radiant energy passing through a vacuum cell and a samplechamber; FIG. 9B showing radiant energy passing through a reference celland the sample chamber; FIG. 9C showing radiant energy passing through ahole and the sample chamber.

FIG. 9D is a graph (1) of the outputs of FIGS. 9A and 9B and the Q-curvefor the ratio of the V_(R) and V_(S) curves (Q=V_(R) /V_(S)); (2) agraph of the outputs of FIGS. 9B and 9C and the U-curve for the ratio ofV_(R) and V_(H) curves (U=V_(R) /V_(H)); and (3) a graph of the outputsof FIGS. 9B and 9C and the U'-curve for the ratio of V_(R) and V_(H')curves (U'=V_(R) /V_(H')). Note that the difference between the outputsV_(H) and V_(H') from FIG. 9C is that in the case of V_(H) there is zeroor negligible CO₂ present in spaces from S to D whereas for the case ofV_(H') there is significant amount of CO₂ (greater than 2 mmHg) presentin spaces from S to D in FIG. 9C.

FIGS. 9E, 9F and 9G are identical to FIGS. 9A, 9B and 9C respectively,with the exception that the sample chamber is replaced by a zero CO₂standard (typically a sapphire blank). Similarly FIGS. 9H, 9J and 9K areidentical to FIGS. 9A, 9B and 9C, respectively, with the exception thatthe sample chamber is replaced by a "span" CO₂ standard cell equivalentto 55 mmHg of CO₂ gas concentration. These arrangements are used tocheck the values for the calibration constants Q₀ and Q₁ in the Double-Qmethod as practiced in the current Hewlett-Packard Capnometer.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, block diagram or box 1 illustrates a patient 20 whois being monitored to measure the CO₂ content of the exhaled breath. Atube 21 is taped or otherwise secured in the vicinity of the patient'snose. The tube leads to a water trap 22 to remove the mucous content andthe other end of the tube 21 is connected to box 2, the sensor section.This connection is brought to a transparent sample chamber SC throughwhich the inhaled and exhaled breath passes, and a flow of inhaled andexhaled air is maintained by a pump 23 to which a tube 24 is connectedto exhaust the air to the exterior of the apparatus.

The apparatus of box or section 2 is provided particularly in accordancewith the invention and has as its novel elements dual chopper wheels W1and W2 rotating in unison, which may be accomplished in several waysincluding a common shaft 26 rotated by a motor 27. The wheels aredescribed with reference to FIG. 2 in more detail. In one part of asingle rotation, radiation passes from a source S through transparentareas in the wheels W1 and W2 and through the transparent sample chamberSC to impinge upon a mirror M2 which can direct the radiation to adetector D directly, or indirectly by means of a semi mirror or beamsplitter BS after traversing filter F.

As the wheels W1 and W2 rotate to a different position in a singlerotation a mirror M5 on wheel W2 intercepts the radiation and directs itdownwardly to a mirror M3 and from there the radiation passes through atransparent section in the bottom of wheel W2 as shown in FIG. 1,through the beam splitter BS, and through a filter F to the detector D.The filter is preferably of the interference type and passes only thedesired radiation, which for CO₂ is preferably in the infrared part ofthe electromagnetic spectrum at about 4.26 microns.

The speed of the rotation of the chopper wheels W1 and W2 is matched tothe characteristics and capabilities of the electronic circuits employedand I presently prefer about 3,000 rpm. The electrical output ofdetector D must be synchronized with the mechanical rotation of thewheels W1 and W2 and this is accomplished by magnetizing parts of one ofthe wheels, or otherwise sensitizing parts and a sync detector 28 isenergized by these portions of wheel W1 to activate the electroniccircuits at the precise intervals.

Considering now box or block 4 of FIG. 1, the analog section, the outputof the detector D of block 2 is delivered to an amplifier 29 and theoutput is delivered by a conductor 31 to an analogue to digitalconverter 32. The conductor 31 is also connected by a wire 33 to thesync detector 28 so that the down stream circuits will be energized atthe correct instants to correspond with the angular positions of wheelsW1 and W2 during a single rotation. The analog section also containscircuits which sample the waveforms generated by the detector D for thevarious angular positions of the wheels W1 and W2 during a singlerotation.

Referring still to FIG. 1, the output from the analog section 4 is fedby conductors 34 and 36 to an input-output logic circuit 35 moreparticularly described as an input logic and logic to output circuit.This is connected to a central processor unit or micro processor CPUlocated in box or block 5, the digital section. The input-output circuit35 (I/O) is also connected to electrically alterable read-only-memoryEAROM, a permanent program read-only-memory ROM, and a volatilerandom-access-memory RAM. The computation takes place in this digitalsection 5 and takes place at every revolution of the chopper wheels W1and W2. The computation is performed to detect changes in the standardCO₂ cell as well as changes of CO₂ content inside the sensor of blockdiagram 2. The electrical waveforms received by the analog to digitalconverter are converted to digital form so that the computation andcomparison can take place in the digital section 5.

The output of the digital section 5 of FIG. 1 may be converted to anydesired form for reading and recording and for illustrative purposessection 6, display, may have a liquid crystal alphanumeric display 38, astrip chart recorder instrument, printer of any desired type or acathode ray tube all designated by the block 39.

The selection of the type of display desired is made by the operator andis designated by the block or box 7. Leading from box 7 are conductors41 and 42 connected to box 6 for selecting the type of display orrecording as well as a conductor 43 leading to the input logic and logicto output circuit I/O of box 5.

Also included in FIG. 1 is a sensor control section 3, having a powersupply 44 connected by conductor 46 to the radiation source S of box 2;having a power supply 47 connected by a conductor 48 to the motor 27 forobtaining extremely uniform speeds of rotation; having a sync detectorcontrol 49 connected to conductor 33; having a pump drive power supply51 connected by conductor 52 to the pump 23; and having a cooler powersupply 60 connected by conductor 50 to the detector D. The PbSe type ofdetector D which I presently prefer requires cooling and, while this canbe done by a refrigerant gas or liquid, I prefer to do it electricallyby using a thermoelectric cooler.

The operation of the apparatus of FIG. 1 is as follows: The electricalsupplies of box 3, sensor control section, are manually turned oncausing the apparatus of box 2 sensor section, to operate. The source Semits radiation which travels through openings in chopper wheels W1 andW2, through the sample chamber SC to the mirror M2 where it is reflectedto beam splitter BS which directs it to the detector D after traversingfilter F. The motor 27 rotates the wheels and the sync detector 28passes signals on conductor 33 to the analog to digital converter 32 ofbox 4 to energize the electronics at the precise instants that thechopper wheels pass radiation. At one part of the rotation of W1 and W2,mirror M5 on W2 will direct radiation to mirror M3, which directs itthrough the beam splitter BS to the detector D after traversing filterF. During operation the pump 23 draws air from the patient 20 throughhose 21 to the sample chamber SC, where its CO₂ content absorbsradiation and this absorption is detected by D. Filter F limits thespectrum to the desired frequency; for example, 4.26 microns formeasuring CO₂.

The output of detector D is amplified at 29 and is in the form of analogpulses which are converted to digital form and directed in synchronismwith the mechanical rotation of wheels W1 and W2 to the digital section5, where measurements are compared to standards and stored, compared tochanging values, and calculations are made by the central processor unitCPU to detect changes in CO₂ gas within the sensor section 2 and anychanges in the gas concentration in the CO₂ standard.

The user selects the type of readout and/or recording he desires byoperating controls at box 7, which can cause alphanumeric readout at 38or a cathode ray tube at 39 or can record by stripchart or computerprintout at 39.

FIG. 2

Illustrated in FIG. 2A is most of the apparatus of box or block 2 ofFIG. 1, the sensor section. Instead of a single shaft for the chopperwheels W1 and W2, there is disclosed two shafts 46 and 47 driven by asingle spur gear 48 to give synchronized rotation of the two chopperwheels. The shafts 46 and 47 may be at 45° to a first optical path 49,but it will be appreciated that both wheels W1 and W2 could be mountedon a single shaft that is parallel to the first optical path 49, and inthis case the mirror M5 will be at an angle on its wheel to deliverradiation to mirror M3. The mirror M3 and detector D define a secondoptical path 51.

Referring now to FIG. 2B, wheel W1 has eight openings numbered 1 through8. Openings 6,7,8 and 1 are occupied by a transparent vacuum cell 52which is an envelope preferably of the same axial thickness as the wheelW1. The envelope is evacuated, or may be filled with a gas that containsno CO₂ and does not absorb radiant energy of the wave band beingutilized. Openings 2,3,4 and 5 of wheel W1 are occupied by a transparentreference cell 53 containing a high concentration of the gas beinganalyzed, for example 380 mmHg of CO₂. Reference cell 53 is alsopreferably of the same axial thickness of wheel W1 and this highconcentration will absorb a large percentage of the radiation as shownby curve V_(R) on FIG. D. V_(R) refers to the voltage at detector D whenradiation passes through the "reference" cell 53.

Referring now to FIG. 2C, wheel W2 also has eight openings numbered 1through 8 in the same relative positions of openings 1 through 8 ofwheel W1 (FIG. 2B). Openings 8,1,2 and 3 are occupied by the mirror M5.Openings 4 and 7 are clear and unobstructed holes 55 through the wheelW2. Openings 5 and 6 are occupied by a transparent standard cell or spancell 54, which is an envelope having CO₂ of a known partial pressure andI presently prefer 55 mmHg or 55 torrs of CO₂. Preferably also, thestandard cell 54 has the same axial width as the wheel W2.

OPERATION OF FIG. 2

The wheels W1 and W2 rotate in unison and the opening 1 of wheel W1 willbe aligned with opening 1 of wheel W2 in the first optical path 49.Successively, the opening pairs 2--2, 3--3, 4--4, 5--5, 6--6, 7--7 and8--8 will be aligned when they are disposed in the first path 49. Thespaces between the wheel openings are opaque, and thus each rotationchops up the radiation from source S into discrete pulses of radiation,and for this reason the wheels W1 and W2 are designated as "chopperwheels". The detector D therefore receives intermittent radiation fromthe source S.

Referring particularly to FIG. 2A when openings 8,1,2 and 3 of wheel W2are disposed in the first path 49 the mirror M5 directs radiation alonga path 56 to the mirror M3. At any of these interceptions by mirror M5,the diametrically opposite openings 4,5,6 and 7 of W2 will transmitradiation because they are either a hole 4 or 7 or are occupied by thetransparent standard cell 54. Therefore, whenever the mirror M5 preventsradiation from reaching the sample cell SC and the mirror M2, the secondpath 51 is receiving radiation. All openings of wheel W1 transmitradiation.

FIG. 2A therefore shows two distinct radiation paths that occur during asingle rotation of the wheels W1 and W2. One path is from source Sthrough openings 4,5,6 and 7 of wheel W1, through openings 4,5,6 and 7of wheel W2, through the sample chamber SC to the mirror M2, to the beamsplitter (semi mirror) BS, through the filter F to the detector D. Theother path is from source S through openings 8,1,2 and 3 of wheel W1, tothe mirror M5 of wheel W2 openings 8,1,2 and 3, to the mirror M3,through openings 4, 5, 6 and 7 in wheel W2, through the beam splitterBS, and through the filter F to the detector D.

FIGS. 4, 5, 6 and 7

The radiation reaching the eight openings of each wheel W1 and W2 arearranged in four pairs to give four separate ratios of V_(R) over V_(S),meaning that in each pair of measurements one measurement is made byradiation passing through the reference cell 53 and the othermeasurement is from radiation passing through the vacuum cell 52, eachin combinations with the sample chamber SC, open holes 4 and 7 of W2 andthe standard cell 54 of W2.

FIG. 4A shows the combination of radiation vacuum cell 52, open hole andsample chamber. FIG. 4B shows the combination of reference cell 53, openhole and sample chamber. In FIG. 4 the sample chamber SC is in theradiation path.

FIG. 5A shows the combination of vacuum cell 52 and standard cell 54.FIG. 5B shows the combination of reference cell 53 and standard cell. InFIG. 5, the standard cell is substituted for the sample chamber which isout of the radiation path.

FIG. 6A shows the combination of vacuum cell 52 and open hole 55 of W2.FIG. 6B shows the combination of reference cell 53 and open hole 55 inW2. In FIG. 6 the sample chamber is out of the radiation path.

FIG. 7A shows the combination of vacuum cell 52, standard cell 54 andsample chamber SC. FIG. 7B shows the combination of reference cell 53,standard cell 54 and sample chamber SC. In FIG. 7 the sample chamber SCis in the radiation path.

FIG. 3

These paths of FIGS. 4, 5, 6 and 7 are tabulated in FIG. 3, which alsonumbers in the left-hand column the eight measurements. In the nextcolumn there is stated the function that each pair of measurementsperforms. In the columns headed W1 and W2 there are stated the openingnumbers of W1 and W2. The "sample cell" column states whether or not thesample cell is in the radiation path, or out of the radiation path. The"measurement" column gives the voltage symbols for each of the eightmeasurements.

Measurements numbers 1 and 2 are performed automatically in the priorart and measurements 3 and 4 are performed manually in the prior art.Measurements 5, 6, 7 and 8 are applicant's novel contribution to the artthat enable the instrument to be self correcting or self-calibrating andwhich also detect any leakage of the standard cell 54.

FIG. 8

Illustrated in FIG. 8 is the sampling of the output of detector D ofFIG. 1 (also FIGS. 2, 4-7). The full sequence of measurement numbers ofthe left-hand column of FIG. 3 is repeated in the horizontal line FIG.8C and there it will be noted that measurement numbers 1, 2 and 3 of thefollowing or next rotation also appear for fuller illustration.

Illustrated in FIG. 8A are representative output voltages from thedetector D. Measurement 1 is a higher wave form than measurement 2because the vacuum cell (FIG. 3) of measurement 1 absorbs less of theradiation than the reference cell of measurement 2. The relative heightsof the other output voltages are also explained with reference to FIG.3.

FIG. 8G illustrates the synchronizing material disposed in differentsectors of periphery wheel W1 so that the position of the two wheels atany instant may be sensed by sync detector 28 of FIG. 1 (box 2). Sector1 of W1 has a split synchronized (or magnetic) material 56 so that itcan be identified, whereas sector 3 has material 57 for the full sector,and sectors 5 and 7 also have full material 57. The output of syncdetector 28 is illustrated in the square wave forms of FIG. 8B and theoutput signal from detector 28 is shown by the cross hatching in FIG.8C.

The sync detector control 49 of FIG. 1 generates a pulse for each sectoras it rotates into the radiation first path 49 (FIG. 2). These aresampling pulses and are shown in FIG. 8D. These sampling pulses causethe sampling of the wave form of FIG. 8A which results in samples closeto the peak of the wave forms of FIG. 8A. The samples are shown incrosshatch in FIG. 8E. The heights or strength of each sample is shownin FIG. 8F. It is these analog heights of FIG. 8F that are converted todigital form by the analog to digital converter 32 of FIG. 1. Thesedigital values are fed into the storage and computing section or box 5of FIG. 1.

RECALIBRATION TECHNIQUE USED IN THE PRESENT INVENTION

Following the double-Q negative filtering technique employed in priorart the pCO₂ in the sample chamber can be represented by a calibrationS-curve in the form:

    pCO.sub.2 (mmHg)=A.sub.0 +A.sub.1 S+A.sub.2 S.sup.2

where S is defined as ##EQU2## where V_(R), V_(S) are the transducedvoltages, numbers 1 and 2 of FIG. 3, at the detector D for the"reference" and "sample" beam, respectively, as described in theNegative Filtering Method. Q₀ is given by ##EQU3## when there is zeroCO₂ flowing through the sample chamber and Q₁ is given by ##EQU4## whenthere is CO₂ of 55 mmHg concentration flowing through the samplechamber.

The quantities A₀, A₁ and A₂ are coefficient constants empiricallydetermined based upon the experimental calibration curve of theinstrument.

THREE ADDITIONAL MEASUREMENT RATIOS

I now define three more Q's as follows:

    Q.sub.S =V.sub.R '/V.sub.S'

as shown in FIG. 5 when the sample chamber is out of the energy path andthe standard cell is in the energy path (hence Q_(S)), and

    Q.sub.A =V.sub.R "/V.sub.S "

as shown in FIG. 6 when there is ambient or negligible (hence Q_(A)) CO₂inside of the instrument enclosure and the sample chamber is out of theenergy path, and

    Q.sub.I =V.sub.S.sup.0 /V.sub.R.sup.0

as shown in FIG. 7 wherein the standard cell and the sample chamber arein series.

As described with reference to FIGS. 1-7, the unique dual-wheel opticalsystem design for the present invention sets up these three measurementratios or conditions, in addition to that used to measure CO₂ via

    Q=V.sub.R /V.sub.S.

One such measurement is shown in FIG. 5 and gives the ratio

    Q.sub.S =V.sub.R '/V.sub.S '.

In this case, an optical arrangement is set up in such a way that thesample chamber SC is temporarily bypassed. In its place is the "span" orstandard cell. Again, a "reference" and a "samle" beam is set up toyield transduced voltages of V_(R) ' and V_(S) ' respectively. Q_(S) issimply defined as the ratio

    V.sub.R '/V.sub.S '.

This arrangement in essence measures an effective CO₂ gas standardrepresented by the pCO₂ content in the sealed standard cell. Inpractice, this CO₂ gas standard can be set approximately at 55 mmHg ofCO₂ gas by adjusting the amount of CO₂ inside the sealed cell duringfilling. Verification can easily be made with the help of the originalcalibration S-curve by substituting S_(S) for S to obtain

    pCO.sub.2 (mmHg)=A.sub.0 +A.sub.1 S.sub.S +A.sub.2 S.sub.S.sup.2

where

    S.sub.S =(Q.sub.S -Q.sub.0)/(Q.sub.1 -Q.sub.0)

and Q₀ and Q₁ are defined earlier.

Another such measurement condition is illustrated in FIG. 6 and givesthe ratio

    Q.sub.A =V.sub.R "/V.sub.S ".

In those wheel positions, an optical arrangement is set up such that thesample chamber is temporarily bypassed. In its place is a volume of theambient environment inside the instrument represented by an open hole inthe dual-wheel assembly. Like the basic measurement for CO₂, a"reference" and a "sample" beam is set up (measurement numbers 5 and 6,FIG. 3). The transduced voltages at the detector are called V_(R) " andV_(S) " respectively, and

    Q.sub.A =V.sub.R "/V.sub.S ".

This particular measurement, in essence, monitors the CO₂ level insidethe instrument. The measured Q_(A) can be used to deduce indirectly theCO₂ concentration inside the instrument by using the calibration S-curvepCO₂ (mmHg)=A₀ +A₁ S_(A) +A₂ S_(A) ², where S_(A), in this case, isgiven by

    S.sub.A =(Q.sub.A -Q.sub.0)/(Q.sub.1 -Q.sub.0)

The third measurement condition is shown in FIG. 7 and gives the ratio

    Q.sub.I =V.sub.R.sup.o /V.sub.S.sup.o.

In this case, an optical arrangement is set up in such a way that thestandard cell and the sample chamber SC line up in series with eachother. Both the "reference" beam and the "sample" beam traverse thisseries combination of standard cell and sample chamber, and thetransduced voltages so obtained at the detector are designated as V_(R)^(o) and V_(S) ^(o) respectively. The ratio Q_(I) is defined as theratio V_(R) ^(o) /V_(S) ^(o) as before. This optical arrangement, ineffect, measures the sum of the CO₂ level in both the standard cell andthe sample chamber. I have discovered that the values of Q and Q_(I) arerelated in a predictable way. I utilize this discovery in the currentinvention to continuously monitor any leakage that might occur in thestandard cell.

UTILIZING THE QS FOR SELF-CALIBRATION

I utilize the simultaneous measurements for the values of Q, Q_(S),Q_(A), and Q_(I) during each dual-wheel revolution for the desirableresult of continuous self-calibration for the current invention for aCO₂ analyzer. Whereas the double-Q negative filtering technique needs toestablish initially two Q values, namely, Q₀ and Q₁, the currentinvention needs to establish two more Q values initially, namely, Q_(A)^(o) and Q_(S) ^(o). Q_(A) ^(o) is defined as the value of Q_(A) whencare is deliberately taken to insure that the CO₂ concentration insidethe instrument is no higher than the ambient CO₂ level, or typically 200ppm. Q_(S) ^(o) is defined as the value of Q_(S) under the samecondition when Q_(A) ^(o) is determined. Like Q₀ and Q₁ both Q_(A) ^(o)and Q_(S) ^(o) are determined experimentally in the beginning and storedaway in EAROM in the computer section 5 of FIG. 1 of the CO₂ instrument.

The value of Q is used along with Q₀ and Q₁ to make the basic CO₂measurement following the double-Q method. The values of Q_(S) and Q_(A)are used to monitor the changes in Q_(S) ^(o) and Q_(A) ^(o) in timeafter their initial determination. The value of Q_(I) is used to monitorany leakage in the span standard cell.

It is important to note that Q_(A) ^(o) ≈Q₀ and Q_(S) ^(o) ≈Q₁. Thissubstantial equality can be effected by tailoring, in the case of Q_(A)^(o), the optical aperture of the open hole in the dual-wheel assemblyand in the case of Q_(S) ^(o) the amount of CO₂ present in the standardcell.

In practice, this equality must be verified by using the originalcalibration S-curve of the instrument. Since the dual-wheel assembly andthe span standard cell are unique to each individual instrument, andhence to the accompanying calibration S-curve, this established equalitybetween Q_(A) ^(o) and Q₀ and Q_(S) ^(o) and Q₁ are self-consistent.

By virtue of the fact that Q_(A) ^(o) ≃Q₀ and Q_(S) ^(o) ≃Q₁ theapplication of detected changes in Q_(A) and Q_(S), namely, ΔQ_(A)=Q_(A) -Q_(A) ^(o) and ΔQ_(S) =Q_(S) -Q_(S) ^(o), directly to Q₀ and Q₁is equivalent to the double-Q recalibration method currently practicedin the Hewlett-Packard CO₂ analyzer. However, since Q_(A) and Q_(S) arebeing continuously monitored for environmental and/or component changesthat could possible alter their values, any detected changes in Q_(A)and Q_(S) can be applied immediately to Q₀ and Q₁ for restoring theinstrument accuracy, thus rendering it continuously self-calibrating.

It can be shown with a little algebra that in the event Q_(A) ^(o) andQ_(S) ^(o) are not exactly equal to Q₀ and Q₁, respectively, then thecorrections to be applied to Q₀ and Q₁ for detected changes in Q_(A) andQ_(S), namely, ΔQ_(A) and ΔQ_(S) where

    ΔQ.sub.A =Q.sub.A -Q.sub.A.sup.o

    ΔQ.sub.S =Q.sub.S -Q.sub.S.sup.o

are given by ##EQU5## and P_(A) and P_(S) are the pCO₂ valuescorresponding to Q_(A) ^(o) and Q_(S) ^(o) (or S_(A) ^(o) =(Q_(A) ^(o)-Q₀)/(Q₁ -Q₀) and S_(S) ^(o) =(Q_(S) ^(o) -Q₀)/(Q₁ -Q₀)) according tothe original calibration S-curve of the instrument, viz.

    pCO.sub.2 (mmHg)=A.sub.0 +A.sub.1 S+A.sub.2 S.sup.2

or

    P.sub.A =A.sub.0 +A.sub.1 S.sub.A.sup.o +A.sub.2 (S.sub.A.sup.o).sup.2

and

    P.sub.S =A.sub.0 +A.sub.1 S.sub.S.sup.o +A.sub.2 (S.sub.S.sup.o).sup.2

It is worth noting that when P_(A) =0, then K₁ =0, and when P_(S) =55mmHg, then K₂ =1. In this case, Eqs. 1 and 2 reduce respectively to ΔQ₀=ΔQ_(A) and ΔQ₁ =ΔQ_(S), as required.

CHECKING THE STANDARD CELL

I have discovered that a simultaneous measurement of Q and Q_(I) enablesthe span or standard cell to be continuously checked for leakage. Theproof of this discovery is as follows. At any instant of time, themeasurement of Q

hence,

    S=(Q-Q.sub.0)/(Q.sub.1 -Q.sub.0)

gives the CO₂ level present in the sample chamber SC. Similarly, themeasurement of Q_(I)

hence,

    S.sub.I =(Q.sub.I -Q.sub.0)/(Q.sub.1 -Q.sub.0)

gives, in effect, the sum of CO₂ present in the standard cell (55 mmHg)and in the sample chamber. Using the calibration curve for theinstrument, we have

    P=A.sub.0 +A.sub.1 S+A.sub.2 S.sup.2                       (3)

where P is the CO₂ level in the sample chamber. Also, we have

    P.sub.S =A.sub.0 +A.sub.1 S.sub.S +A.sub.2 S.sub.S.sup.2   (4)

where P_(S) is the effective pCO₂ in the standard cell and

    S.sub.S =(Q.sub.S.sup.o -Q.sub.0)/(Q.sub.1 -Q.sub.0)

is the corresponding quantity obtained from Q_(S) ^(o). Note that Q_(S)^(o) (and, hence, S_(S)) is stored as a calibration constant and isretrievable from the computer memory on demand. Furthermore,

    P+P.sub.S =Q.sub.0 +A.sub.1 S.sub.I +A.sub.2 S.sub.I.sup.2 (5)

where

    S.sub.I =(Q.sub.I -Q.sub.0)/(Q.sub.1 -Q.sub.0)

Thus, by adding Equations (3) and (4), and comparing the sum withEquation (5), one has

    A.sub.2 S.sub.I.sup.2 +A.sub.1 S.sub.I +C=0                (6)

where C=-[A₀ +A₁ (S+S_(S))+A₂ (S² +S_(S) ²)]

Equation (6) can be solved to yield ##EQU6## In equation (7), everythingis known (A₀, A₁, A₂, are stored in non-volatile ROM and S and S_(S) arecalculated from the measured values of Q and the stored value of Q_(S)^(o) (in EAROM), respectively.

Thus, by checking the self-consistency of Equation (7) above, namely bycomparing the calculated value for S_(I) using Equation (7) with S_(I)actually being measured (FIG. 7), one can readily deduce whether S_(S)stays constant as it should be. The calculated value for S_(I) shouldagree with that being measured if there is no leak in the span standardcell. In this fashion, there is a continuous check of the standard cellfor leakage, which is important because the S-curve calibration is basedupon the gas content of the standard cell.

CO₂ CALCULATION AND SELF-CALIBRATION USING THE COMPUTER

The digital electronics (FIG. 1, box 5) needed to generate the CO₂output value and to accomplish the continuously self-calibratingfunction of the current invention for a CO₂ analyzer consists of amicroprocessor, volatile random-access-memory (RAM), permanent programread-only-memory (ROM), a non-volatile but electrically alterable ROM(EAROM), and input logic and logic to output information to the user 35(I/O).

The original calibration S-curve for the instrument is permanentlystored in ROM in the form of three coefficient constants A₀, A₁, and A₂.These coefficient constants are experimentally and a priorily determinedaccording to procedures that are well-known to those knowledgeable inthe art of CO₂ gas analyzers. Four additional parameters, Q₀, Q₁, Q_(S)^(o), and Q_(A) ^(o), which are similarly determined, are stored inEAROM.

If either Q_(A) ^(o) ≠Q₀ or Q_(S) ^(o) ≠Q₁, then two additional initialparameters K₁ and K₂ (see section under "Utilizing the Qs forSelf-Calibration") are also stored in ROM. The values of K₁ and K₂depend upon the values of Q_(A) ^(o) and Q_(S) ^(o) and the originalcalibration S-curve for the instrument as explained in the sectioncited. The manner by which they are determined is also well-known tothose knowledgeable in the business of manufacturing CO₂ gas analyzers.

In addition, three constants, ΔI, ΔA, and ΔS, are stored in ROM. Thevalue of ΔI is used to determine whether the span standard cell isleaktight. The values of ΔA and Δs are used to determine if thecharacteristics for the components inside the instrument have changed tothe extent, due either to aging or external stimuli, that arecalibration is needed to restore the performance accuracy of theinstrument.

Under normal operation a new set of Q values, namely, Q, Q_(A), Q_(S),and Q_(I) are fed, during each dual-wheel revolution, into the CentralProcessing Unit (CPU) of the microprocessor via the input logic 35 fordata processing.

CHECKING FOR LEAKAGE IN SPAN STANDARD CELL

Using the input values of Q and Q_(I) and retrieving the values of Q_(S)^(o), Q₀ and Q₁ stored in EAROM, the CPU computes the values for S,S_(S), and S_(I) according to the relations already cited, viz.

    S=(Q-Q.sub.0)/(Q.sub.1 -Q.sub.0)

    S.sub.S =(Q.sub.S.sup.o -Q.sub.0)/(Q.sub.1 -Q.sub.0)

and

    S.sub.I =(Q.sub.I -Q.sub.0)/(Q.sub.1 -Q.sub.0)

Using the values of S and S_(S) just computed and further retrieving thevalues of A₀, A₁, and A₂ from ROM, the CPU calculates a value S_(I) *,which is the expected value for S_(I) if the span standard cell isleaktight according to the formula: ##EQU7##

The CPU next calculates the absolute value of S_(I) -S_(I) *, namely,|S_(I) -S_(I) *|, and checks whether this value exceeds ΔI, which isretrieved from ROM.

If |S_(I) -S_(I) *| exceeds ΔI, then the span standard cell is leakingCO₂ gas. (Note that in actual practice this detected inequality must beallowed to repeat itself several times in succession before themicroprocessor declares that the span standard cell is not leaktight.)In the event that the span standard cell is actually verified to beleaking CO₂ gas, an "inoperative" sign is instructed to be displayed onthe display panel of the instrument (FIG. 1, box 6) via the output logicof the digital electronics (FIG. 1, box 5).

If |S_(I) -S_(I) *| is less than ΔI, then the span standard cell isleaktight and the microprocessor proceeds to next programmed function.

CHECKING FOR SHIFTS IN Q_(A) ^(o) AND Q_(S) ^(o)

Using the input values of Q_(A) and Q_(S) and retrieving the values ofQ_(A) ^(o) and Q_(S) ^(o) from EAROM, the CPU computes ΔQ_(A) and ΔQ_(S)using the formulae:

    ΔQ.sub.A =Q.sub.A -Q.sub.A.sup.o

and

    ΔQ.sub.S ×Q.sub.S -Q.sub.S.sup.o

Next the CPU checks the magnitudes for ΔQ_(A) and ΔQ_(S) by comparingrespectively their absolute values with the values ΔA and ΔS stored inROM. If either |ΔQ_(A) |>ΔA or |ΔQ_(S) |>ΔS, then the instrumentrequires recalibration. Otherwise, the CPU is ready to calculate the CO₂output values.

RECALIBRATION OR SELF-CALIBRATION

To do this, the CPU recalls the values for K₁ and K₂ from ROM andtogether with the values of ΔQ_(A) and ΔQ_(S) just calculated computesthe values for ΔQ₀ and ΔQ₁ according to the formulae: ##EQU8## Thecalculated values of ΔQ₀ and ΔQ₁ are added or subtracted from the valuesof Q₀ and Q₁ retrieved from EAROM dependent upon the sign of ΔQ₀ and ΔQ₁to form the new Q₀ and Q₁ values, viz.

    Q.sub.0.sup.N =Q.sub.0 +ΔQ.sub.0

    Q.sub.1.sup.N =Q.sub.1 +ΔQ.sub.1

The old set of Q₀, Q₁, Q_(A) ^(o), and Q_(S) ° values are next erasedfrom the EAROM, and a new set of Q values comprising Q₀ ^(N), Q₁ ^(N),Q_(A), and Q_(S) are now stored in their place in EAROM. Note that thepreviously stored values for Q_(A) ^(o) and Q_(S) ^(o) are beingreplaced by the most current Q_(A) and Q_(S) values, respectively.

CO₂ OUTPUT VALUE CALCULATION

The CPU next retrieves the most recent values of Q₀ and Q₁ (note thatthese values may have just been updated) from EAROM and together withthe input value of Q calculates the parameter S according to:

    S=(Q-Q.sub.0)/(Q.sub.1 -Q.sub.0)

It then retrieves the values of A₀, A₁, and A₂ from ROM and calculatesthe CO₂ value according to:

    pCO.sub.2 (mmHg)=A.sub.0 +A.sub.1 S+A.sub.2 S.sup.2

This will be the pCO₂ value outputted to the display panel for displayvia the output logic (FIG. 1, box 5). In the following claims thesymbols used are defined as:

Q is V_(R) /V_(S)

Q₀ is V_(R) /V_(S) when there is no CO₂ in the sample chamber(determined empirically)

Q₁ is V_(R) /V_(S) when a known concentration of CO₂ is flowed throughthe sample chamber, such as 55 mmHg.

Q_(S) is V_(R) '/V_(S) ' when sealed standard cell of knownconcentration is substituted for the sample chamber.

Q_(A) is V_(R) "/V_(S) " when an open hole in the chopper wheel issubstituted for the standard cell and the sample chamber is out of thepath.

Q_(A) ^(o) is the value of Q_(A) when the CO₂ concentration inside theinstrument is that of ambient air (empirically determined)

Q_(S) ^(o) is the value of Q_(S) when the CO₂ concentration inside theinstrument is that of ambient air (empirically determined)

Q_(I) is V_(R) ^(o) /V_(S) ^(o) when the standard cell and the samplecell are in series.

S=(Q-Q₀)/(Q₁ -Q₀)

S_(S) =(Q_(S) -Q₀)/(Q₁ -Q₀)

S_(A) =(Q_(A) -Q₀)/(Q₁ -Q₀) and

: S_(I) =(Q_(I) -Q₀)/(Q₁ -Q₀)

A₀, A₁ and A₂ are coefficients empirically determined to represent thecalibration S-curve and are stored in ROM

S-curve is pCO₂ (mmHg)=A₀ +A₁ S+A₂ S²

ΔQ_(A) =Q_(A) -Q_(A) ^(o)

ΔQ_(S) =Q_(S) -Q_(S) ^(o)

ΔI stored, is the selected tolerance for standard cell leakage

ΔA stored, is the selected tolerance for CO₂ inside the instrument

ΔS stored, is the selected tolerance for other component changesaffecting the standard cell pCO₂ value

K₁ is ##EQU9## K₂ is ##EQU10## S₁ *= ##EQU11## The following are theelectrical changes of the detector when radiant energy passes throughthe following elements:

V_(S) : vacuum cell and open hole and sample cell,

V_(R) : reference cell and open hole and sample cell,

V_(S) ': vacuum cell and standard cell,

V_(R) ': reference cell and standard cell,

V_(S) ": vacuum cell and open hole,

V_(R) ": reference cell and open hole,

V_(S) ^(o) : vacuum cell and open hole and sample cell,

V_(R) ^(o) : reference cell and standard cell and sample cell.

I claim:
 1. Optical-mechanical apparatus for a gas analyzer forgenerating electrical signals for continuously and automaticallyself-calibrating the electrical output comprising:(a) a first chopperwheel containing openings occupied by two items of (1) vacuum cell, (2)reference cell, (3) open hole, (4) standard cell; (b) a second chopperwheel containing openings occupied by the other two items of element "a"above; (c) means for aligning the openings of the two wheels to define apath; (d) means for rotating the two wheels in synchronism to alignselected openings in the two wheels along said path; (e) a source ofradiant energy disposed at one end of said path; (f) a sample chamberdisposed along said path downstream from said wheels; (g) an electricaltransducer detector of radiant energy disposed at the other end of saidpath to create changes in electrical quantities,whereby the detectorcreates values V_(S), V_(R), V_(S) ', V_(R) ', V_(S) ", V_(R) ", V_(S)^(o), and V_(R) ^(o), for use in rendering the gas analyzerself-calibrating, and wherein these values are: V_(S) : vacuum cell andopen hole and sample cell, V_(R) : reference cell and open hole andsample cell, V_(S) ': vacuum cell and standard cell, V_(R) ': referencecell and standard cell, V_(S) ": vacuum cell and open hole, V_(R) ":reference cell and open hole, V_(S) ^(o) : vacuum cell and standard celland sample cell, V_(R) ^(o) : reference cell and standard cell andsample cell.
 2. Apparatus as set forth in claim 1 wherein a filter isdisposed in the path to pass only a selected portion of theelectromagnetic spectrum.
 3. Apparatus as set forth in claim 1 whereinthe two wheels are mounted on the same axle.
 4. Optical-mechanicalapparatus for a gas analyzer for generating electrical signals forcontinuously self-calibrating the analyzer output comprising:(a) asource of radiant energy radiating energy along a first path; (b) anelectrical transducer detector for receiving energy at one end of asecond radiant energy path; (c) a first mirror disposed at the other endof said first path and directing energy on said detector; (d) a secondmirror disposed at the other end of said second radiant energy path; (e)a first chopper wheel containing openings and rotatable to disposeselected openings in said first path, said openings occupied by twoitems of (1) vacuum cell, (2) reference cell, (3) open hole, (4)standard cell; (f) a second chopper wheel containing openings occupiedby the other two of said items of "e" above and also containing mirrorsand rotatable to dispose selected openings on both paths, said chopperwheel mirrors being so disposed that energy in the first path isdirected to the second mirror in the second path; (g) a sample chamberdisposed in said first path downstream from the chopper wheels; and (h)means for synchronizing the rotation of the two wheels to disposeselected openings simultaneously in the two paths;whereby said detectorgenerates electrical signals of V_(S), V_(R), V_(S) ', V_(R) ', V_(S) ",V_(R) ", V_(S) ^(o), and V_(R) ^(o), and wherein these signals resultfrom radiant energy passing through the following wheel openings: V_(S): vacuum cell and open hole and sample cell, V_(R) : reference cell andopen hole and sample cell, V_(S) ': vacuum cell and standard cell, V_(R)': reference cell and standard cell, V_(S) ": vacuum cell and open hole,V_(R) ": reference cell and open hole, V_(S) ^(o) : vacuum cell andstandard cell and sample cell, V_(R) ^(o) : reference cell and standardcell and sample cell.
 5. Apparatus as set forth in claim 4 wherein abeam splitter is disposed in the second path downstream from the secondchopper wheel and the first mirror directs energy on the beam splitterto thereby direct energy from the first path to the detector. 6.Apparatus as set forth in claim 4 wherein a filter is disposedimmediately upstream from the detector.
 7. In a gas measurementapparatus employing sample chamber, a reference cell and a vacuum cellto obtain V_(R) /V_(S) measurement, a standard cell for calibration anda computer, the method of detecting leakage in the standard cellcomprising:(a) storing in permanent memory at least three numericalcoefficients A₀, A₁, and A₂ to represent the calibration S-curve; (b)storing in nonvolatile but erasable memory Q₀, Q₁, and Q_(S) ^(o) ; (c)continuously measuring V_(R) when the reference cell and the samplingchamber are in series; (d) continuously measuring V_(S) when the vacuumcell and the sampling chamber are in series; (e) continuously ratioingsaid V_(R) over said V_(S) to obtain Q(Q=V_(R) /V_(S)); (f) continuouslymeasuring V_(R) ^(o) when the reference cell, standard cell, andsampling chamber are in series; (g) continuously measuring V_(S) ^(o)when the vacuum cell, standard cell, and sampling chamber are in series;(h) continuously ratioing said V_(R) ^(o) over said V_(S) ^(o) to obtainQ_(I) (Q_(I) =V_(R) ^(o) /V_(S) ^(o)); (i) continuously calculating S,S_(S), and S_(I) from the measured values of Q and Q_(I) and Q₀, Q₁ andQ_(S) ^(o) values retrieved from erasable memory; (j) storing inpermanent memory a constant value ΔI representing a tolerance betweenthe measured S_(I) (calculated directly from Q_(I)) and the calculatednew S_(I) or S_(I) *; (k) continuously calculating S_(I) * in accordancewith the formula ##EQU12## (l) calculating the difference between S_(I)and S_(I) *; (m) comparing the absolute value of the difference betweenS_(I) and S_(I) * to the tolerance ΔI; and (n) energizing an alarm aftera selected time delay if the absolute value of the difference betweenS_(I) and S_(I) * exceeds the tolerance ΔI.
 8. In a gas measurementapparatus employing a sample chamber, a reference cell and a vacuum cellto obtain a V_(R) /V_(S) measurement, a standard cell for calibrationand a computer, the method of checking for shifts in Q_(A) ^(o) andQ_(S) ^(o) comprising:(a) storing in nonvolatile but erasable memoryQ_(A) ^(o) and Q_(S) ^(o) ; (b) storing in permanent memory a toleranceΔA; (c) storing in permanent memory a tolerance ΔS; (d) continuouslymeasuring Q_(A) and Q_(S) ; (e) continuously computing a new ΔQ_(A)according to the formula

    ΔQ.sub.A =Q.sub.A -Q.sub.A.sup.o

(f) continuously computing a new ΔQ_(S) according to the formula

    ΔQ.sub.S =Q.sub.S -Q.sub.S.sup.o

(g) continuously comparing ΔQ_(S) with ΔS stored and comparing ΔQ_(A)with ΔA stored; and (h) continuously correcting Q_(A) ^(o) and Q_(S)^(o) by ΔQ_(A) and ΔQ_(S), respectively, in erasable memory if eitherΔQ_(S) or ΔQ_(A) exceeds the tolerances.
 9. In a gas measurementapparatus employing a sample chamber, a reference cell and a vacuum cellto obtain V_(R) /V_(S) measurement, a standard cell for calibration anda computer having erasable memory and permanent memory, the method ofcontinuously self-calibrating the S-curve as components change withaging, temperature, and other causes, comprising:(a) storing A₀, A₁, A₂,ΔA, ΔS, K₁, and K₂ in permanent memory; (b) storing Q₀, Q₁, Q_(S) ^(o),and Q_(A) ^(o) in nonvolatile but erasable memory; (c) continouslymeasuring and calculating Q_(A) and Q_(S) ; (d) continuously computingΔQ_(A) according to the formula ΔQ_(A) =Q_(A) -Q_(A) ^(o) ; (e)continously computing ΔQ_(S) according to the formula ΔQ_(S) =Q_(S)-Q_(S) ^(o) ; (f) continously comparing ΔQ_(A) with ΔA stored; (g)continously comparing ΔQ_(S) with ΔS stored; (h) if either ΔQ_(A) orΔQ_(S) exceeds the tolerance ΔA or ΔS respectively, continuouslycomputing ΔQ₀ according to the formula ##EQU13## (i) if either ΔQ_(A) orΔQ_(S) exceeds the tolerance ΔA or ΔS respectively, continuouslycomputing ΔQ₁ according to the formula ##EQU14## (j) algebraicallyadding calculated ΔQ_(A) from stored Q_(A) ^(o) to obtain Q_(A) ^(o),new and store the result in erasable memory; (k) algebraically addingcalculated ΔQ_(S) from stored Q_(S) ^(o) to obtain Q_(S) ^(o), new andstore the result in erasable memory; (l) continuously calculating Q₀^(new) according to the formula

    Q.sub.0.sup.new =Q.sub.0 +ΔQ.sub.0

(m) continuously calculating Q₁ ^(new) according to the formula

    Q.sub.1.sup.new =Q.sub.1 +ΔQ.sub.1

(n) erasing from memory the prior values of Q₀, Q₁, Q_(A) ^(o), andQ_(S) ^(o) and substituting the values of Q₀ ^(new), Q₁ ^(new), Q_(A)^(o), new, and Q_(S) ^(o), new ; and (o) continuously calculating Svalues from measured Q values using Q₀ ^(new) and Q₁ ^(new) according tothe formula

    S=(Q-Q.sub.0.sup.new)/(Q.sub.1.sup.new -Q.sub.0.sup.new)


10. In a gas measurement apparatus employing a sample chamber, areference cell and a vacuum cell to obtain a V_(R) /V_(S) measurement, astandard cell for calibration and a computer, the method of calculatingthe partial pressure of the gas comprising:(a) storing in permanentmemory the values of A₀, A₁ and A₂ representing the S calibration curve;(b) continuously measuring, correcting, and storing in erasable memoryQ₀ and Q₁ ; (c) continuously computing Q from the formula Q=V_(R) /V_(S)(d) continuously calculating S from the formula

    S=(Q-Q.sub.0)/(Q.sub.1 -Q.sub.0)

(e) calculating the partial pressure of the gas from the formula

    pCO.sub.2 (mmHg)=A.sub.0 +A.sub.1 S+A.sub.2 S.sup.2

(f) and delivering the result to an intelligible display device.